From 240c879f20a605346705be24253bc9fc6fa8a106 Mon Sep 17 00:00:00 2001 From: Minchan Kim Date: Thu, 13 Jan 2011 15:46:27 -0800 Subject: [PATCH] writeback: avoid unnecessary determine_dirtyable_memory call I think determine_dirtyable_memory() is a rather costly function since it need many atomic reads for gathering zone/global page state. But when we use vm_dirty_bytes && dirty_background_bytes, we don't need that costly calculation. This patch eliminates such unnecessary overhead. NOTE : newly added if condition might add overhead in normal path. But it should be _really_ small because anyway we need the access both vm_dirty_bytes and dirty_background_bytes so it is likely to hit the cache. [akpm@linux-foundation.org: fix used-uninitialised warning] Signed-off-by: Minchan Kim Cc: Wu Fengguang Cc: Peter Zijlstra Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/page-writeback.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/mm/page-writeback.c b/mm/page-writeback.c index 28763b8..2cb01f6 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -410,9 +410,12 @@ void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty) { unsigned long background; unsigned long dirty; - unsigned long available_memory = determine_dirtyable_memory(); + unsigned long uninitialized_var(available_memory); struct task_struct *tsk; + if (!vm_dirty_bytes || !dirty_background_bytes) + available_memory = determine_dirtyable_memory(); + if (vm_dirty_bytes) dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE); else -- 2.7.4